# Bash 的 export 命令

export NAME[=VALUE]...

$ export NAME=Primers   # 创建一个新的变量并导出

$ DOMAIN=xplanc.org
$ export DOMAIN         # 导出一个已经存在的变量

$ export -p
declare -x DOMAIN="xplanc.org"
declare -x NAME="Primers"
declare -x OLDPWD
declare -x PWD="/"
declare -x SHLVL="1"

EXPORT(1P)              POSIX Programmer's Manual              EXPORT(1P)
PROLOG         top
       This manual page is part of the POSIX Programmer's Manual.  The
       Linux implementation of this interface may differ (consult the
       corresponding Linux manual page for details of Linux behavior), or
       the interface may not be implemented on Linux.
NAME         top
       export — set the export attribute for variables
SYNOPSIS         top
       export name[=word]...

       export -p
DESCRIPTION         top
       The shell shall give the export attribute to the variables
       corresponding to the specified names, which shall cause them to be
       in the environment of subsequently executed commands. If the name
       of a variable is followed by =word, then the value of that
       variable shall be set to word.

       The export special built-in shall support the Base Definitions
       volume of POSIX.1‐2017, Section 12.2, Utility Syntax Guidelines.

       When -p is specified, export shall write to the standard output
       the names and values of all exported variables, in the following
       format:

           "export %s=%s\n", <name>, <value>

       if name is set, and:

           "export %s\n", <name>

       if name is unset.

       The shell shall format the output, including the proper use of
       quoting, so that it is suitable for reinput to the shell as
       commands that achieve the same exporting results, except:

        1. Read-only variables with values cannot be reset.

        2. Variables that were unset at the time they were output need
           not be reset to the unset state if a value is assigned to the
           variable between the time the state was saved and the time at
           which the saved output is reinput to the shell.

       When no arguments are given, the results are unspecified.
OPTIONS         top
       See the DESCRIPTION.
OPERANDS         top
       See the DESCRIPTION.
STDIN         top
       Not used.
INPUT FILES         top
       None.
ENVIRONMENT VARIABLES         top
       None.
ASYNCHRONOUS EVENTS         top
       Default.
STDOUT         top
       See the DESCRIPTION.
STDERR S         top
       The standard error shall be used only for diagnostic messages.
OUTPUT FILES         top
       None.
EXTENDED DESCRIPTION         top
       None.
EXIT STATUS         top
        0    All name operands were successfully exported.

       >0    At least one name could not be exported, or the -p option
             was specified and an error occurred.
CONSEQUENCES OF ERRORS         top
       Default.

       The following sections are informative.
APPLICATION USAGE         top
       Note that, unless X was previously marked readonly, the value of
       "$?" after:

           export X=$(false)

       will be 0 (because export successfully set X to the empty string)
       and that execution continues, even if set -e is in effect. In
       order to detect command substitution failures, a user must
       separate the assignment from the export, as in:

           X=$(false)
           export X
EXAMPLES         top
       Export PWD and HOME variables:

           export PWD HOME

       Set and export the PATH variable:

           export PATH=/local/bin:$PATH

       Save and restore all exported variables:

           export -p > temp-file
           unset a lot of variables
           ... processing
           . temp-file
RATIONALE         top
       Some historical shells use the no-argument case as the functional
       equivalent of what is required here with -p.  This feature was
       left unspecified because it is not historical practice in all
       shells, and some scripts may rely on the now-unspecified results
       on their implementations. Attempts to specify the -p output as the
       default case were unsuccessful in achieving consensus.  The -p
       option was added to allow portable access to the values that can
       be saved and then later restored using; for example, a dot script.
FUTURE DIRECTIONS         top
       None.
SEE ALSO         top
       Section 2.14, Special Built-In Utilities

       The Base Definitions volume of POSIX.1‐2017, Section 12.2, Utility
       Syntax Guidelines
COPYRIGHT         top
       Portions of this text are reprinted and reproduced in electronic
       form from IEEE Std 1003.1-2017, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The
       Open Group Base Specifications Issue 7, 2018 Edition, Copyright
       (C) 2018 by the Institute of Electrical and Electronics Engineers,
       Inc and The Open Group.  In the event of any discrepancy between
       this version and the original IEEE and The Open Group Standard,
       the original IEEE and The Open Group Standard is the referee
       document. The original Standard can be obtained online at
       http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page
       are most likely to have been introduced during the conversion of
       the source files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                2017                        EXPORT(1P)